In previous work on this project, the investigators have analyzed the effect of calcium intake on calcium retention, and net skeletal accretion, in white adolescent females aged 12 to 14 years. They also have analyzed the effect of age on calcium retention and kinetics in girls studied longitudinally, at one level of calcium intake. The first Aim of the currently proposed study is to determine the interaction of calcium intake and age, in order to develop a model to individualize the calcium intake required for maximal calcium retention. Calcium balance and kinetics, and predictors of bone turnover, will be determined in girls aged 15 to 17 years who were previously studied at age 12 to 14 years on a range of calcium intakes between 800-1860 mg/day. Optimizing calcium accretion during this rapid period of growth should maximize peak bone mass within the genetic potential and reduce risk of osteoporosis later. The second Aim of the proposed study is to compare differences in calcium metabolism and bone turnover, biochemical markers of bone turnover, mediators of calcium absorption and bone density, between black and white adolescent girls. The greater bone density in blacks versus whites is largely unexplained. Although markers of bone turnover are lower in blacks than whites, neither net bone accretion, nor calcium retention has been determined in blacks. Understanding underlying racial differences may provide the means for attaining higher bone mass. Stable isotopes of calcium will be administered orally, and by intravenous injection, after a one week equilibrium period during a three week metabolic camp. Analysis of complete urine and fecal collections and periodic blood samples will provide data for multicompartmental and stochastic analysis of calcium metabolism. Bone mass and total body and skeletal site calcium measured by dual energy X-ray absorptiometry, hormone levels (PTH, vitamin D metabolites, IGF-1) and biochemical markers of bone turnover (serum osteocalcin and bone-specific alkaline phosphatase, and urinary hydroxyproline and collagen crosslinks) will be determined each year to correlate accumulation of bone mass with the rate of bone remodeling. The relationship of calcium balance and biomarkers to menstruation and ovulation will also be studied.